1. Field of the Invention
The present invention proceeds from a method having the features defined in the preamble of claim 1.
2. Description of the Related Art
A method of this kind has been known from WO 97/00784. That document discloses a method for operating a device for monitoring and indicating a pressure change in vehicle tires by wire, being arranged in the tire as one module together with a valve, and comprising a current source, a pressure sensor arranged for measuring the tire pressure at first time intervals, an analog/digital converter for digitizing a pressure signal obtained from the pressure sensor, a memory for storing the pressure signal, a transmitter for transmitting the measured tire pressure information to a receiving unit located in the vehicle, and a comparator, especially one realized in one module with a microprocessor or a similar microelectronic processing unit. The comparator compares each pressure signal that has been picked up and transmitted by the pressure sensor with a previously stored comparison pressure signal. The last pressure signal measured, or a mean value derived from a plurality, for example from three, measured pressure signals is used as the reference pressure signal. The comparator controls the transmitter in such a way that the transmitter will transmit signals at second time intervals greater than the first time intervals as long as the decrease of the pressure signal relative to the reference pressure signal (referred to herein as pressure signal drift or, simply, as drift) does not exceed a threshold value during a predetermined time interval, but will transmit signals at third time intervals smaller than the second time intervals when and as long as the drift exceeds the pressure threshold value. This operating mode is based on the consideration that the by far greatest part of tire punctures that are accompanied by a pressure loss are due to, or announce themselves by, a slow xe2x80x9ccreepingxe2x80x9d pressure loss. In a practical embodiment of a device for monitoring tire pressure
the first time intervals are 3 s,
the second time intervals are 54 s,
and the drift threshold value at which a change-over occurs from the slow transmission rate (time intervals of 54 s) to the rapid transmission rate at third time intervals is 0.2 bar/min.
The third time intervals are only 0.8 s with the known device; these are the short time intervals at which measurements are taken and transmitted in the case of the known device when the drift exceeds the threshold value, in order to be able, in any case, to detect and signal any dangerous pressure loss at an early point in time. With the measuring and transmission rate that can be changed over in the described way, a battery service life of 7 to 8 years has been reached in such electronic devices at wheels. There is, however, a demand for an additional extension of the battery service life. From another commercially available device for monitoring and signaling tire pressure by radio it has been known to provide a centrifugal switch in the form of a reed switch as part of the wheel electronics, which switch is closed when the respective wheel turns at a speed of at least approximately 25 km/h so that the wheel electronics will be activated only when the vehicle is in motion and that speed is exceeded. The wheel electronics therefore consumes no battery current in the stationary condition of the vehicle. The system is, however, connected with the disadvantage that the centrifugal switch, being an electromechanical component, is prone to failure so that reliable operation over many years cannot be guaranteed. A further disadvantage lies in the fact that no tire pressure monitoring is effected at low driving speed, during times of traffic congestion and during stop-and-go moving conditions and that there is no stable speed threshold beyond which monitoring takes place. In addition, current consumption under moving conditions is still excessively high also with these other available wheel electronics.
Now, it is the object of the present invention to further extend battery service life in a device for monitoring the tire pressure of the before-mentioned kind, without impairing the safety and reliability of the current tire pressure monitoring operation and without sacrificing the tire pressure monitoring operation in the stationary condition of the vehicle.
This object is achieved by a method having the features defined in claim 1. Advantageous further developments of the invention are the subject-matter of the sub-claims.